Orbital refilling

Transferring propellant between spacecraft in orbit so a vehicle can depart for the Moon or Mars with full tanks.

• Reaching low orbit uses most of a rocket's propellant, leaving too little for the large burns needed to go further.
• Tanker flights of the same reusable rocket top up a depot or the departing ship, replacing one enormous launch with many smaller ones.
• NASA's Artemis Moon landings depend on it; estimates of the tanker flights needed per Starship landing range from about 10 to 20.
• It has never been done at this scale: in weightlessness liquids don't settle, super-cold propellants boil off, and transfers must be automatic.